1. Field of the Invention
The present invention relates to an illumination device used, among others, as a backlight of a liquid crystal display device and to a liquid crystal display device provided with the illumination device.
2. Description of the Related Art
In recent years, liquid crystal display devices have gained in popularity at a rapid pace in place of cathode ray tubes or CRTs. Features of liquid crystal display devices include low energy consumption, thinness and light weight, and they are widely used in flat-screen televisions, monitors and portable phones, among others. To further enhance the features of liquid crystal display devices, it is an option to ameliorate an illumination device disposed at a backside of a liquid crystal display device.
Illumination devices are mostly divided into direct illumination devices and edge-light-type illumination devices. In direct illumination devices, a plurality of linear light sources (such as cold cathode tubes or hot cathode tubes) or a plurality of spot-shaped light sources such as light-emitting diodes are provided in a line at a backside of a liquid crystal panel. The light outputted from the light sources is directly irradiated onto the liquid crystal panel. With this configuration, a direct illumination device can more easily achieve a high luminance, even in the case of a large screen. Accordingly, direct illumination devices are mostly used as backlights of large-sized (twenty inches and over) liquid crystal displays.
Edge-light-type illumination devices include a light guide at the backside of the liquid crystal panel, as well as a light source on the side edge. The light outputted from the light source is reflected by the light guide and indirectly irradiated onto the liquid crystal panel. A positional relationship of the light source and of the light guide is determined by positioning carried out by utilizing a shape of the housing. In recent illumination devices designed for portable phones, especially, the positional relationship of the light source and of the light guide is also determined by positioning carried out by utilizing double-side tape in addition to the shape of the housing. With the edge-light-type illumination device having this configuration, the luminance is low; however, it is possible to achieve a thin shape, and homogeneity of the luminance is excellent. As a result, edge-light-type illumination devices are mostly used as backlights of medium-sized and small display devices such as portable phones or notebook computers.
Under such circumstances, in recent years, tandem-type illumination devices have been disclosed, in which a plurality of light guides are disposed so as to be aligned and stacked at the back of a liquid crystal display panel (as for example in Japanese Patent Application Publication, Tokukaihei, No. 11-288611 (Publication Date: Oct. 19, 1999) and Japanese Patent Application Publication, Tokukai, No. 2001-75096 (Publication Date: Mar. 23, 2001)). With this configuration, it is possible to obtain an illumination device having both the merits of direct-type illumination devices (i.e., high luminance and large size) and the merit of edge-light-type illumination devices (i.e., thinness).
In the case of an edge-light-type illumination device or of a tandem-type illumination device, a positional relationship of a light source and of a light guide wields a great influence over a brightness of the illumination device. For example, in the case that the light source and the light guide are disposed extremely close to each other, a light incidence efficiency of the light guide increases, thus making it possible to obtain an illumination device having a high light use efficiency. On the other hand, in the case that the light source and the light guide are disposed slightly apart from each other, the light incidence efficiency of the light guide decreases, thus resulting in a problematic decrease of the brightness of the illumination device. This problem becomes more severe as the distance between the light source and the light guide is further increased.
Further, when providing the light guide (whose function is to diffuse the light emitted from the light source) with a hole to allow the passage of a fixation member (such as a screw) in the direction of movement of the light, the light becomes diffused in a different direction than in other areas of the light guide. As a result, the luminance of the illumination device is no longer homogeneous.
In addition, in a tandem-type illumination device, a plurality of light guide units composed by assembling together a light source and alight guide are disposed in parallel and in stacks, in order to form a large-sized illumination device. Therefore, the following problems arise in the case that each of the light guide units disposed in parallel and in stacks is constructed by assembling together a light source and a light guide in different positional relationships. Namely, the brightness for each light guide unit is different, and the luminance homogeneity of the illumination device degrades considerably.